The present invention relates to a filtering method and apparatus for removing contaminants from coolants employed in metal forming operations.
Prior to the development of precision metal drawing and ironing techniques, beverage and food cans were made from three pieces of metal. That is, a cylinder was formed from sheet metal stock by joining two ends to form a side seam which was bonded by crimping, welding or soldering. The top and bottom of a can were formed independently and sealed into place.
Through the use of advanced drawing and ironing techniques the two-piece can was developed. This type can incorporates the sidewalls and a formed bottom in one piece, with the top being an independent sealing member. It is with the process of drawing and ironing that the instant invention concerns itself. More particularly, this specification describes a method and apparatus for treating liquid coolants used in drawing and ironing operations for the manufacture of metal cans. Of course, however, utility of the present invention is not limited to drawing and ironing operations, but is useful in any situation where the contaminants to be filtered are electrically charged and are too small for removal by ordinary filtering or settling techniques.
A two-piece can is manufactured by first stamping out the sidewall and bottom portion in a cup-like form from a sheet of metal. This metal sheet may be aluminum, tin-free steel, or tinplate. The second operation performed on the cup-like sidewall and bottom portion is called drawing and ironing. This operation forms the bottom of the can in a deep draw over a mandrel, with ironing ring tools making the deep draw possible by keeping the metal sidewall thickness uniform.
Shaping sheet metal material into the two-piece can by drawing and ironing causes friction between the mandrel tool and the metal cup, which may be reduced by using a liquid lubricant. This lubricant, commonly called a coolant also serves to dissipate heat from the working surface and the metal cup. The coolant serves one further purpose, that is to carry away any dirt particules, metal fines, metal oxides, or tramp oil or greases which find their way into the system.
There are two primary sources of liquid coolant contaminants. The first source is that of the metal itself which is being drawn and ironed. The surface of the metal sheet is coated with a very thin layer of metal oxides. As the metal is drawn and ironed these metal oxides along with pure metal fines flake off or are scraped off the stretched metal cup. These fines, which are near colloidal in size, are carried away from the machining operation by the liquid coolant. There are also present a certain amount of metal and carbides worn from tool surfaces.
The second major source of liquid coolant contaminants is that of tramp oils and greases which find their way into the liquid coolant as leakage from the stamping, drawing and ironing machines in the form of lubricating and hydraulic oils and greases.
These contaminants build up in the liquid coolant as it is recirculated repeatedly through the can forming system. Unless adequately treated, the concentration of contaminants will reach such a point that it will become necessary to dispose of the coolant and begin with a new batch.
It is suspected that a considerable part of the particulate contaminants as first generated are very fine. However, as these particulate contaminants are circulated again and again through the operating system they are probably ground finer and finer between the mandrel tools and metal working surfaces. The average diameter size of these particulate contaminants is on the order of 100 to 500 angstrom units. Because the particulate contaminants are so fine it is practically impossible to remove them from the coolant using present filtration or other treatment methods.
Thus the particulate contaminants are approaching a collidal size. Thousands of these particules will find their way into a single droplet of tramp oil suspended in the liquid coolant. The fineness of the particulate contaminants and the suspension of tramp oils creates a condition which in the past has defied ordinary techniques for treating and conditioning liquid coolants. A filter media which is tight enough to entrap the suspended contaminants surface loads and yields unreasonably short filtration cycles. Filter media which is open enough not to be affected by the surface loading characteristics of tramp oils does not exhibit any degree of effective particle removal. Standard settling, without chemical treatment, is practically ineffective since the suspended contaminants have a specific gravity very near that of the liquid coolant thus resulting in little or no clarification from settling techniques. Chemical treatment destroys the liquid coolant and is thus out of the question.